1. Field of the Invention
The present invention relates to a semiconductor variable capacitance diode.
2. Description of Related Art
A MOS Varactor having a MOS structure and a P-N junction diode having a P-N junction structure have been known as a semiconductor variable capacitance diode. The present invention relates to the latter.
Said P-N junction diode comprises a laminated body consisting of a P-type semiconductor layer 1 and an N-type semiconductor layer 2 and electrodes 4, 5 provided on both sides of said laminated body, as shown in FIG. 1. The P-N junction diode is used with a backward voltage applied, as shown in the drawing. A junction capacitance of this P-N junction diode is changed depending upon said applied voltage, as shown in FIG. 2. Said capacitance is inversely proportional to a square root of the voltage. It is the reason why the capacitance is changed that an extension of a depletion layer of P-N junction is dependent upon the applied voltage. Since said extension of said depletion layer resulting from a change of the applied voltage is limited, a ratio of a change of the capacitance to said change of the voltage can not be increased. In order to solve this problem, for example, a device, in which a gradient of a distribution of concentration of impurities in said P-type semiconductor layer 1 and said N-type semiconductor layer 2 is steepened (density the junction side) has been adopted but it has not been sufficient.
In addition, though it is desirable for a variable capacitance diode to have a large performance index Q, Q can not be increased because the application of backward voltage naturally increases its series resistance.
Furthermore, the application of backward voltage naturally produces a small leakage current.